(i) Field of the Invention
The present invention relates to a novel process for preparing a methyl methacrylate polymer, and more specifically, it relates to a process for stably and efficiently preparing a high-quality methyl methacrylate polymer.
(ii) Description of the Related Art
Methyl methacrylate polymers have excellent characteristics with respect of transparency, weathering resistance, mechanical strength and appearance, and therefore they have been used not only as various molding materials such as building materials, furniture and interior decoration materials, car parts and electric parts, but also as optical materials and optical communication materials.
Heretofore, as a preparation method of the methyl methacrylate polymer which has been used as a molding material, a suspension polymerization method has usually been employed, but in this method, the polymer is contaminated with a secondary material such as a suspending dispersant, and for this reason, it is difficult to obtain a high-purity polymer. The polymer manufactured by this method is not suitable for an optical material or the like. Additionally, an after-treatment step for the polymer comprises intricate operations of filtration, washing and drying, and a large amount of waste water must be treated. Thus, the suspension polymerization method is still insufficient as an industrial process. As techniques which can overcome the drawbacks of this suspension polymerization method, much attention has been paid to a continuous mass polymerization method and a solution polymerization method in recent years. These methods permit the manufacture of high-quality resins having excellent optical properties, because the suspending dispersant or the like is not used.
However, in the continuous mass polymerization method is very difficult owing to an autoacceleration effect called "the Trommsdorff effect" which threatens the polymerization reaction being stably controlled, while a high monomer conversion is maintained, and hence the final polymer having a high polymer concentration is scarcely obtained.
Japanese Patent Publication No. 32665/1977 (U.S. Pat. No. 3,900,453) has suggested a continuous mass polymerization method in which a uniform phase reaction is carried out at a relatively low monomer conversion by the use of a tank type reactor and unreacted monomer is continuously separated and removed. In this method, the unreacted monomer remains in large quantities, and for the collection and reuse of the unreacted monomer and for the concentration of the obtained polymer, a large amount of energy must be consumed in a volatilization step. In this case, since the polymer is subjected to a heat history, coloring and modification tend to adversely occur.
On the other hand, as shown in Japanese Patent Publication No. 7845/1980, it is known that according to the solution polymerization method in which benzene or an alkylbenzene is used as a solvent, the viscosity of a reaction solution decreases by the solvent, so that the Trommsdorff effect is inhibited and a stable polymerization reaction is possible at a high monomer conversion.
In the case of such a solution polymerization method, the decrease in the amount of the solvent to be used is limited, and even if the amount of the unreacted monomer which remains in the reaction solution is reduced by increasing the monomer conversion, the content of volatiles inclusive of the solvent cannot be decreased. In consequence, the energy consumption required to remove the volatiles is equal to the case of the mass polymerization method on occasion. In addition, there are problems such as the deterioration of the thermal decomposition resistance of the polymer and the complication of procedures for collecting and reusing the monomer component.
On the other hand, Japanese Patent Application Laid-open No. 241905/1987 discloses a method in which a monomer mixture containing methyl methacrylate as a main component in an aliphatic monovalent alcohol such as methanol as a solvent is subjected to radical polymerization, and the resulting polymer is precipitated in the state of a slurry and then separated. In this method, however, the solvent must be used in a large amount, i.e., in an amount of 80 to 30 parts by weight with respect to 20 to 70 parts by weight of the monomer component, and a precipitation step, a filtration step and a drying step are necessary to separate the polymer. In addition, in the case that this method is utilized as a continuous process, there are many problems which should be industrially solved to uniformly transfer and treat the polymerization solution in the slurry state.
Furthermore, Japanese Patent Application Laid-open No. 201307/1989 discloses a method in which the solution polymerization of a monomer mixture containing methyl methacrylate as a main component is carried out using not less than 5% by weight and less than 30% by weight of a mixed solvent comprising a monovalent alkylalcohol and benzene or an alkylbenzene. In this method, however, the monovalent alkylalcohol merely occupies 5 to 50% by weight of the mixed solvent, and so the method is not such as to fundamentally change a usual method using benzene or the alkylbenzene as the solvent.
In the meantime, the methyl methacrylate polymer begins to decompose from about 230.degree. C., and its decomposition becomes noticeable at about 270.degree. C. On the other hand, this methyl methacrylate polymer can be injection-molded or extruded at 230.degree. C. to 250.degree. C. At this time, the molding temperature of the methyl methacrylate polymer is close to the temperature of its thermal decomposition, so that the monomers formed by the thermal decomposition of the polymer remain in the molded article, which leads to practical problems such as the generation of silver streaks and voids, coloring, the decline of heat deformation resistance and the deterioration of operation circumstances by odor.
In order to improve the thermal decomposition resistance of the methyl methacrylate polymer, various contrivances have been heretofore made. For example, it has been tried that an antioxidant is added at an early stage, followed by heating and molding, but in this case, a sufficient effect cannot be obtained and on the contrary, a drawback such as coloring is observed.
In recent years, it has been disclosed that the methyl methacrylate polymer is manufactured by a continuous polymerization method to improve the thermal decomposition resistance. For example, the above-mentioned Japanese Patent Publication No. 32665/1977 discloses a method where in performing one-stage complete stirring tank type continuous polymerization at a temperature of 130.degree. to 160.degree. C., there is continuously fed a monomer composition which contains 0.01 to 1.0 mol % of a mercaptan as a chain transfer agent and which meets the following formulae, to maintain a monomer conversion at 50 to 78%: EQU 10.gtoreq.A.sup.1/2 .multidot.B.sup.-1/2 .times.10.sup.3 EQU 3.gtoreq.A.multidot.B.times.10.sup.5 EQU 2.9.gtoreq.A.sup.-1 .multidot.(B+10.3).times.10.sup.-6
wherein A is mols of a radical polymerization initiator in 100 g of the fed monomer; and B is a half-value period (hours) of the radical polymerization initiator at its polymerization temperature.
Moreover, Japanese Patent Publication No. 111408/1991 discloses a method where in performing the one-stage complete stirring tank type continuous polymerization, an initiator having a half-value period of 0.5 to 2 minutes at a polymerization temperature of 130.degree. to 160.degree. C. is used, and an average residence time is set so that a ratio of the half-value period of a radical polymerization initiator at the polymerization temperature to the average residence time may be in the range of 1/200 to 1/10,000, whereby a monomer conversion becomes 45 to 70%.
In these methods, the thermal decomposition resistance in question has been evaluated for the polymers subjected to an after-treatment step such as a vacuum volatilization step for removing residual volatiles such as the unreacted monomer at a high temperature or an extrusion step.
According to investigation by the present inventors, the thermal decomposition resistance of the polymer itself produced in the polymerization step is not always sufficient. In view of the deterioration of yield by the thermal decomposition in an after-treatment step such as the vacuum volatilization step or the extrusion step and the coloring by the heat history, it is very important to improve the thermal decomposition resistance of the polymer produced in the polymerization step.
Furthermore, inventions regarding multi-stage complete stirring tank type continuous polymerization have also been disclosed.
In Japanese Patent Publication Laid-open No. 172401/1989 (U.S. Pat. No. 4,728,701), it is described that characteristics such as thermal stability of the produced polymer can be improved by dividing and feeding parts of a comonomer such as methyl acrylate or ethyl acrylate and a chain transfer agent, but any detailed description about this conception has not been disclosed in examples.
In addition, it has been elucidated from experiments of the present inventors that when the methyl methacrylate polymer having a molecular weight suitable for a molding material is prepared in accordance with the method described in the above-mentioned Japanese Patent Application Laid-open No. 241905/1987, a polymer having an extremely low thermal decomposition resistance is obtained.